1) Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile, or a printer. More specifically, the present invention relates to an image forming apparatus that electrostatically transfers a toner image from an image carrier to a paper or the like.
2) Description of the Related Art
Image forming apparatus such as copying machines, facsimiles, or printers are widely known. Mainly two types of transfer devices are used in the image forming apparatuses. In the first type, a toner image is electrostatically transferred from an image carrier to a roller member and then the image from the roller member is transferred to a recording element. Generally the image carrier is a photosensitive element, the roller member is a transfer roller, and the recording element is a transfer paper. In the second type, the toner image is directly electrostatically transferred from the image carrier to transfer paper. In the second type, the recording element is held between the photosensitive element and the roller member when transferring the toner image to it.
In recent years, there is an increasing demand for better image quality. The image quality can be increased by improving dot reproducibility. To improve the dot reproducibility, it is necessary to use a toner (sometimes “toner” is used to mean toner particles) that is finer, more spherical, more uniform, and that can be charged more uniformly, than the conventional toner. Polymer toner is an example of such a toner. Because the polymer toner is almost spherical, it has low aggregation. In other words, because non-electrostatic adhesion force between toner particles of the polymer toner is small, the toner particles can easily and smoothly move on the transfer paper. Therefore, uniform developing can be achieved, and smooth halftone images can be obtained with the polymer toner. However, because the polymer toner has low aggregation, there is a problem that the polymer toner easily gets transformed into transfer dust.
When a toner image is electrostatically transferred from a transfer source to a transfer target, some toner particles fly off and result into the transfer dust.
One of causes of the transfer dust to occur includes an abrupt change in an electric field or a peel discharge phenomenon that occurs near the entrance/exit of a transfer nip formed at a contact point between the transfer element and the photosensitive element.
Japanese Patent Application Laid Open (JP-A) No. 2000-221800 discloses a technology of preventing transfer dust by providing a pushing roller that pushes an intermediate transfer belt from its inner peripheral side against a photosensitive drum at a contact nip between the photosensitive drum and the intermediate transfer belt to increase toner aggregation.
JP-A No. 2001-209255 discloses a technology of suppressing transfer dust by defining a volume resistivity of the transfer target as 108 Ω·cm to 1014 Ω·cm, a linear velocity ratio between the transfer source and the transfer target as 0.85 to 1.10, a nip pressure as 5 g/cm2 or higher, toner aggregation as 3% to 15%, and an apparent density as 0.35 g/cm3 to 0.50 g/cm3.
JP-A No. H9-062028 discloses a technology of suppressing transfer dust by setting an amount of coat with toner on a developer carrying element to 0.5 mg/cm2 to 1.5 mg/cm2.
When a difference in rotational speed occurs between the photosensitive element and the transfer roller, shearing force occurs between the photosensitive element and the transfer paper. If the aggregation of toner particles is low, a toner layer cannot accommodate the shearing force, which easily causes occurrence of a phenomenon such that the toner layer collapses. The phenomenon is so-called “transfer blur” such that the collapse causes a transferred image to blur. Particularly, when a ratio of an image area in the toner layer is higher, the transfer blur occurs more easily. To solve this problem, the photosensitive element and the transfer roller are desired to rotate at a speed perfectly equal to each other. JP-A No. H9-062028 describes a technology of causing the transfer roller to rotate, from a drive source of the photosensitive element through a gear, at a speed equal to that of the photosensitive element.
JP-A No. 2001-115425 describes a technology of defining a position and a contact pressure of a transfer roller. JP 2000-221800 A discloses a technology of pushing a floating roller against a photosensitive element. JP-A No. 2001-209255 discloses a technology of defining volume resistivity of an intermediate transfer element and physical property of toner.
JP-A No. H7-005776 discloses a technology of applying transfer bias to a pushing roller using an amorphous-silicon photosensitive element and using capsule toner as toner. Furthermore, JP-A No. H9-062028 discloses, in order to achieve both prevention of voids in characters and improvement of printing accuracy, a technology of improving printing accuracy by rotating the transfer roller at a speed equal to that of the photosensitive element, and a technology of preventing voids in characters as a side effect by using toner characteristics.
However, if the transfer roller is driven from a drive source of the photosensitive element via gears and a belt, the transfer roller cannot be made to rotate at a speed perfectly equal to that of the photosensitive element. This is caused by changes in torque due to engagement of gear teeth, and slack or deflection of the belt, which may cause the transfer blur to occur.
The quality of images in the conventional electrophotographic system is greatly inferior to that of printed images such that granularity as an important index of high image quality is 0.3 or higher. The granularity is expressed by an average value of 40 to 80 in average luminance, explained later. In order to obtain high image quality having a granularity of 0.25 or lower, it is required to improve degradation in images called as transfer dust, blur as an blurred image, or an uneven toner image that is obtained as a result of transferring insufficient toner to a transfer element (hereinafter, “uneven toner” or “uneven toner image”), occurring in a transfer process. However, the conventional technology has difficulty in achieving a granularity of 0.25 or lower.